1. Field of the Invention
The invention relates to a method for detection of an electromagnetic signal, which is transmitted from a transmitting antenna, by means of at least two at least essentially identical receiving antennas whose sensitivity curve has a maximum with falling flanks as well as sidelobes adjacent to it with sensitivity increased again at a reception angle symmetrically with respect to a basic alignment, with angle determination for an object which reflects the transmitted signal being carried out by the two receiving antennas by phase determination in an unambiguity area whose boundaries are predetermined by the distance between the receiving antennas.
The invention also relates to an apparatus having a transmitting antenna for transmission of an electromagnetic signal, and at least two essentially identical receiving antennas, whose sensitivity curve has a maximum with falling flanks as well as sidelobes adjacent to it with sensitivity increased again at a reception angle symmetrically with respect to a basic alignment, and having an evaluation device for determination of the phase differences of the signal which is received by the two receiving antennas and has been reflected by an object, in an unambiguity area which is predetermined by the distance between the receiving antennas.
2. Background Description
It is known for the angle of an object which reflects the transmission signal to be determined by determination of the phase offset between two received signals received by receiving antennas. The so-called phase monopulse method (Merrill I. Skoinik, Radar Handbook, Second Edition, McGraw Hill 1990, pages 18-9 et seqq. and 18-17 et seqq.) is illustrated in FIG. 1. The phase difference Δφ corresponds to a path length λ/2π·Δφ. When the two receiving antennas are aligned in the same way, the relationship is sin θ=λ/2π·Δφ/d.
The angle θ is thus given by:
  θ  =      arcsin    ⁢                  (                  λ          ⁢                                          ⁢          Δ          ⁢                                          ⁢          Φ                )                    2        ⁢                                  ⁢        π        ⁢                                  ⁢        d            
The angle measurement is ambiguous when the phase difference is Δφ≧π or Δφ≦−π. The area in which there is no ambiguity is the unambiguity area (Huder “Einführung in die Radartechnik” [Introduction to radar engineering] 1999, pages 146 to 148). Systems such as these are normally operated at radar wavelengths. A wavelength of λ=12.43 mm and a distance d=14.55 mm between the receiving antennas results in an unambiguity area of θu=±25.39°.
For an unambiguity area which is as large as possible, the distance d would have to be as small as possible. However, this would also result in very small receiving antennas which would not allow adequate beam formation of the received signals. For beam formation that is as good as possible, the receiving area of the receiving antennas must be chosen to be as large as possible. However, this would result in an unusably small unambiguity area. The sidelobes of the receiving antennae, in which the receiving antennas once again have increased sensitivity and thus produce strong signals from objects at the side which can interfere with the detection of useful signals within a relatively narrow angular range around the basic alignment of the receiving antennas, have a particularly disturbing effect on the evaluation of the received signals, that is to say in particular on the determination of the location and/or the speed of the object. This applies in particular to radar systems on motor vehicles which, for example, are operated at 24 GHz and are used, for example, as automatic proximity sensors for vehicles traveling in front, generally in the same lane.